This invention relates to driver circuits for emitter switched gate turn-off silicon controlled rectifier (SCR) devices.
The turn-off characteristics of a gate controlled switch is one of the main considerations in the design of a power electronics circuit. It affects not only the circuit efficiency but also system reliability. The turn-off characteristics of a transistor depend not only on the transistor itself but also on the turn-off scheme used. Conventionally, a bipolar power transistor is turned off by reverse biasing the base-emitter junction. In general, the larger the reverse base current is, the faster the transistor turns off. However, danger may arise in the turn-off of an inductive load (such as an AC-DC inverter supplying a motor), in that the transistor may suddenly lose voltage blocking capability due to second breakdown. This in turn usually leads to the destruction of the transistor.
A turn-off scheme referred to as emitter-open or emitter switched turn-off in which the main power transistor is in series with a low voltage high speed transistor and the turn-off of the main power transistor is accomplished by opening the main power transistor emitter terminal circuit, i.e. turning off the low voltage high speed transistor, provides superior transistor turn-off characteristics as compared with conventional reverse-biased turn-off.
As pointed out by William F. Wirth in his IEEE Industrial Applications Society, Conference Record paper at pages 788-793 entitled "The Gate Turn-off Thyristor In the Cascode Configuration" delivered at the 1983 Annual Meeting, using the emitter open or cascode connection of a bipolar transistor at bus voltages over 600 volts begins to reveal some of limitations of the bipolar transistor; namely poor forward biased safe operating area, low gain, and deficiencies in the current carrying capabilities of metallization and wire bonds in the base area and speed up diode. The gate turn-off thyristor (GTO) is a good candidate to overcome these difficulties because it is similar in many ways to the bipolar transistor but has a number of advantages inherent in a four layer device that prove useful in high voltage (greater than 600 volt) applications.
The low voltage, high speed bipolar transistor used in an emitter switched gate turn-off SCR device configuration requires significant base drive to obtain low collector-emitter voltage drop when the bipolar transistor is biased into conduction. The base drive current is normally supplied by a dedicated floating power supply which adds circuit complexity and cost to present emitter switched gate turn-off drives.
It is an object of the present invention to substantially reduce the base drive current supplied by the power supply in emitter switched gate turn-off drivers.
It is another object of the present invention to reduce the required number of capacitors and eliminate all inductors in an emitter switched gate driver circuit so that the circuit is suitable for hybrid circuit/power module implementation.